Spent nuclear fuel generated in a nuclear power plant contains nuclides mainly comprising unburnt .sup.235 U and .sup.239 Pu formed from .sup.238 U contained in the fuel in addition to fission products, though they vary depending on the composition of the fuel, burnup and cooling time.
Recently, spent nuclear fuel is reprocessed by finely shearing it and chemically processing the sheared fuel to recover reusable uranium and plutonium in order to effectively use uranium resources. A high-level radioactive liquid waste generated in the course of this reprocessing contains fission noble metals such as ruthenium (Ru), palladium (Pd) and rhodium (Rh) in amounts of as high as one tenth of the whole fission products.
After being reprocessed with nitric acid, these fission noble metals are contained in both high-level radioactive liquid waste and solid particles (hereinafter referred to as the insoluble residue) mainly comprising platinum group elements difficultly soluble in nitric acid. The recovery of the fission noble metals from the high-level radioactive liquid waste and the insoluble residue is regarded as important from the viewpoint of the use of rare metals as the resource, and industrial methods for selectively separating these noble metals from the high-level radioactive liquid waste and the insoluble residue have been developed and proposed.
For example, a method for recovering the noble metals from the high-level radioactive liquid waste is reported in G. A. Jansen et al., Nuclear Technology, Vol. 65, May (1984), Preliminary Report of 1989 Fall Meeting of the Atomic Energy Society of Japan, etc. Further, a method for recovering the noble metals from the insoluble residue is reported in Japanese. Patent Laid-Open Specification No. 4-106499 (1992) and K. Naito, T. Matsui and T. Tanaka, Journal of Nuclear Science and Technology, Vol. 23, 6 (1986).
It is reported that the fission noble metals are efficiently recovered in a lump by these methods. However, the recovered noble metals include ruthenium having a particularly high radioactivity and palladium and technetium both having a long half-life and, therefore, these platinum-group elements must be separated from each other in order to separate and recover the most valuable rhodium. Thus, the development of such a technique is eagerly demanded.
Investigations have been made hitherto on the separation of the platinum-group elements and so on from each other by the solvent extraction, ion exchange, adsorption, precipitation, electrolytic reduction or distillation method. However, these methods are substantially unsuitable for the practical use, because high-degree mutual separation of the elements is insufficient.